Fix issues found by FindBugs: SBSC_USE_STRINGBUFFER_CONCATENATION

http://findbugs.sourceforge.net/bugDescriptions.html#SBSC_USE_STRINGBUFFER_CONCATENATION

Change-Id: I498d219e98cc821b4b9096697356cf54f7e033ce
diff --git a/src/main/java/net/onrc/onos/core/intent/runtime/PlanInstallRuntime.java b/src/main/java/net/onrc/onos/core/intent/runtime/PlanInstallRuntime.java
index 7607373..00c3f7e 100644
--- a/src/main/java/net/onrc/onos/core/intent/runtime/PlanInstallRuntime.java
+++ b/src/main/java/net/onrc/onos/core/intent/runtime/PlanInstallRuntime.java
@@ -84,14 +84,16 @@
         }
 
         static void printCount() {
-            String result = "FLOWMOD COUNT:\n";
+            StringBuilder result = new StringBuilder();
+
+            result.append("FLOWMOD COUNT:\n");
             for (FlowModCount count : map.values()) {
-                result += count.toString() + '\n';
+                result.append(count.toString() + '\n');
             }
             if (map.values().isEmpty()) {
-                result += "No flow mods installed\n";
+                result.append("No flow mods installed\n");
             }
-            log.error(result);
+            log.error(result.toString());
         }
     }
 
@@ -153,5 +155,4 @@
         // TODO: we assume that the plan installation succeeds for now
         return true;
     }
-
 }